1. Field of the Invention
The present invention relates to an apparatus and method for improving pulverizer operation and reducing NO.sub.x formation by burning fuel in the primary air supply line.
2. Description of the Prior Art
In the combustion of coal to raise steam and for other purposes, it is common to first pulverize the coal to a very finely divided size and blow the coal into the furnace as a powder suspended in air and then burn the coal in a flame similar to an oil or natural gas flame. The coal is typically pulverized until most of it is less than 40 micrometers in size. The coal comes from the ground with moisture in it. During storage and transportation it may rain on the coal, and as a result additional moisture may be present in the coal. The moisture prevents efficient pulverization of the coal. To dry the coal, hot air is fed into the pulverizers and the hot air flow then sweeps the milled coal from the pulverizers. When the hot air flow is in the pulverizer, it is in intimate contact with the unmilled and partially milled coal. The flow of the hot air across the coal dries the coal. The partially milled coal is easier to dry than the feed coal since it has a higher surface to volume ratio than the feed coal. During the drying process, the hot air is cooled because much of its energy is given up to evaporate the moisture in the coal. Typically, the incoming air is 400.degree. to 600.degree. F. and the pulverized coal and primary air exit the pulverizer at 135.degree. to 165.degree. F. The large amount of power used to drive the pulverizer ultimately shows up as heat in the coal and primary air, but both that and much of the heat in the incoming air goes to dry the coal.
Excessively wet coal is difficult to handle. At times the drying of the coal is not adequate even though the coal may not be excessively wet. The air is usually heated in an air heater which takes heat from gas leaving the economizer. Often the same air heater used to heat the secondary air is used to heat the primary air. The primary air may be 20% of the total air or 2.5 pounds per pound of coal; however, these ratios can be different in different units and with different coals. If more heat is needed to dry the coal it is often not possible to simply increase the temperature of the incoming air with the prior art air heater.
Using a greater amount of air would supply more energy so that coal could be dried. However, the increased volume would change the cut size on the classifier. Such a change will cause smaller coal sizes to be returned to the pulverizer for further milling which may overload the pulverizers. Also, the increased air volume would increase the flow velocity in the primary pipes which increases the erosion of these pipes. The increased velocity could also change the flame shape or even cause the flame to go out.
Increasing the incoming air temperature would assist in drying the coal. However, the increased air temperature will increase the tendency for fires and explosions in the pulverizer because both the inlet and the outlet temperature would be increased. Increasing the volume increases the exit temperature and thus also increases the tendency for fires and explosions. These events cause millions of dollars in loss of equipment and lost production every year. Typically, one U.S. worker is killed by a pulverizer explosion every year. Even with the constraints of today's operation, there are very important losses due to pulverizer fires and explosions.
Another method of increasing the temperature in the pulverizers and thus improving the drying of the coal is to mix hot flue gas with the incoming air and thereby increase the temperature. This has the advantage of replacing some of the oxygen in the incoming air with other gases. It also has several disadvantages. The flue gas will usually contain some sulfur trioxide (SO.sub.3) which will form corrosive sulfuric acid by reacting with water at the temperature of the pulverizer and the acid will corrode the pulverizer, the coal pipes and the burner. The flue gas will contain flyash and this will erode the pulverizer and especially the high pressure fan needed to blow the flue gas into the pulverizer or the exhauster which extracts the gas and coal from the pulverizer.
La Haye and Bjerklie in U.S. Pat. No. 3,868,211 disclose an improved combustion device where flue gas or combustion products are recirculated with the secondary air. While they have made an improvement, it is clearly for natural gas or for oil fuel. It is well known that recirculation of combustion products in the secondary air of a coal-fired surface has little effect on the NO.sub.x emissions. Hence, the process as defined in U.S. Pat. No. 3,868,211 would not reduce NO.sub.x from a coal-fired boiler more than a very small amount. Furthermore, if U.S. Pat. No. 3,868,211 were used on a coal-fired boiler it would not improve the pulverizer operation since the secondary air does not go through the pulverizer.
Nitrogen oxide (NO.sub.x) emissions from coal-fired power plants cause considerable damage to the environment. Once discharged to the atmosphere they are converted to nitrogen dioxide (NO.sub.2) and this gas is toxic. The gas is brown or red and contributes to the discoloration of air over many cities. The NO.sub.2 reacts with water to form nitric acid (HNO.sub.3) and the HNO.sub.3 is a major component of acid rain. In addition, the NO.sub.x is an important contributor to the major urban air containment, ozone. As a result there are many federal, state, and local limits on the emissions of NO.sub.x from coal-fired power plants. Many devices and techniques for NO.sub.x reduction have been invented and put in use. However, more reductions are needed since the existing NO.sub.x reduction devices are expensive and do not always reach the legal limits.
Thus, equipment and processes which will safely improve coal pulverizer operations, and method and apparatus which will reduce NO.sub.x emissions from coal-fired furnaces are needed.